A few years ago, the card used to take months after approval to arrive. Nowadays for most people the card comes within a couple of weeks, so getting the stamp is usually unnecessary, and they will often refuse to give you a stamp if they expect the card will be sent to you soon. You normally have to be waiting for at least 30 days without receiving the card before they will stamp your passport.
Note that if you have advance parole, you can use it to reenter the US at least for a short time post-approval if you didn't receive the card nor a passport stamp before leaving the US. I'm not sure how much time post-GC the AP can be used, but I know of people having done it for a trip of a month without a problem.